The present invention relates generally to the construction of mirrors and more particularly relates to a prestressed mirror and a method for fabricating the same.
High concentration solar thermal power systems typically rely on a field of heliostats, or a parabolic dish or trough concentrators to track the sun and reflect solar radiation to a receiver where the solar energy heats a working fluid, such as steam. The working fluid is then employed to provide thermal energy for various industrial and commercial processes or to produce electricity. Similarly, concentrating photovoltaic systems use mirrors of varying types to collect solar energy where it is turned directly into electrical energy.
In such systems, it is critical for performance objectives that the mirror facets which make up these systems meet stringent optical performance characteristics such as radius of curvature, reflectivity and surface slope error. It is also critical that these mirror facets be lightweight so as to reduce the cost associated with the drive units that are needed to aim the mirror facets. The mirror facets must also be sufficiently robust to ensure a long life despite their exposure to precipitation, wind and sun. Consequently, these mirror facets must be capable of withstanding sustained winds in excess of 100 m.p.h., temperatures ranging from xe2x88x9240xc2x0 F. to 130xc2x0 F., impacts from hail, corrosive elements (e.g., acid rain, salt), humidity changes, etc. Furthermore, as there may be hundreds or even thousands of mirror facets in a system, it is highly desirable that the mirror facet be of highly cost efficient construction.
The designs of conventional mirror facets have relied on the thickness of the glass that forms the mirror facet and/or the frame structure of the mirror facet to compensate for the relatively weak tensile properties of glass. This approach has several drawbacks, including losses in reflectivity as a result of the use of relatively thicker glass and a relatively higher weight. Additionally, these mirror facets are not as robust as desired, being highly susceptible to damage during shipping, installation and use. Furthermore, as these mirror facets have relatively weak tensile properties, their exposure to time-varying forces such as wind can cause the propagation of cracks which could permit the reflective finish of the mirror facet to corrode, with the result being impaired performance of the mirror facet.
In view of these drawbacks, some conventional mirror facets have obtained additional strength through the use of operations: such as slumping, chemical strengthening, annealing and/or tempering. These processes tend to be relatively expensive, and as such, a substantial cost penalty is incurred if these processes are employed. Furthermore, these mirror facets typically rely on relatively thicker glass and as such are accompanied by drawbacks such as losses in reflectivity and higher weight.
It is one object of the present invention to provide a glass structure which is robust yet light in weight and relatively inexpensive to manufacture.
It is another object of the present invention to provide a glass structure which is robust yet utilizes a relatively thin glass member.
It is yet another object of the present invention to provide a glass structure which employs a structure that applies a compressive force to a glass member to place the glass member in compression as to improve the strength of the glass structure.
It is a further object of the present invention to provide a glass structure which employs a relatively lightweight reinforcing member that does not affect the surface slope error of the glass structure.
It is yet another object of the present invention to provide a method for forming a glass structure.
In one preferred form, the present invention provides a glass structure having a glass member, a composite structure and a support structure. The composite structure includes a rigid interlayer which is bonded to the glass member and exerts a compressive force thereon to place the glass member in compression. The support structure is used to mount the glass structure and prevents the glass member from collapsing due to the compressive force exerted by the rigid interlayer.
In another preferred form, the present invention provides a method for forming a glass structure comprising the steps of providing a glass member and securing a rigid interlayer to the glass member such that the rigid interlayer applies a compressive force to the glass member.